Method of making uranium dioxide bodies

ABSTRACT

Sintered uranium dioxide bodies having controlled density are produced from U3O8 and carbon by varying the mole ratio of carbon to U3O8 in the mixture, which is compressed and sintered in a neutral or slightly oxidizing atmosphere to form dense slightly hyperstoichiometric uranium dioxide bodies. If the bodies are to be used as nuclear reactor fuel, they are subsequently heated in a hydrogen atmosphere to achieve stoichiometry. This method can also be used to produce fuel elements of uranium dioxide plutonium dioxide having controlled density.

United States Patent [191 I Wilhelm et al.

[11] hen 145] Sept, 25, M73

[ METHOD OF MAKING URANIUM DIOXIDE BODIES [75] Inventors: Harley A.Wilhelm; James K.

McClusky, both of Ames, lowa [22] Filed: May 22, 1972 [21] Appl. No.:255,603

[56] References Cited UNITED STATES PATENTS 6/1963 Langrod 423/2618/1969 Robertson..... 252/301.1 R 10/1970 Norman et a1.... 264/0.52/1972 Horsley et a1 264/05 3,320,179 5/1967 Gens 252/301.I S

OTHER PUBLICATIONS Belle, Uranium Dioxide: Properties and NuclearApplications, 1961, USAEC Published, pp. 7374 Primary Examiner-Carl D.Quarforth Assistant Examiner-R. L. Tate Attorney-Roland A. Anderson [57]ABSTRACT Sintered uranium dioxide bodies having controlled density areproduced from U 0 and carbon by varying the mole ratio of carbon to U 0in the mixture, which is compressed and sintered in a neutral orslightly oxidizing atmosphere to form dense slightly hyperstoichiometricuranium dioxide bodies. If the bodies are to be used as nuclear reactorfuel, they are subsequently heated in a hydrogen atmosphere to achievestoichiometry. This method can also be used to produce fuel elements ofuranium dioxide plutonium dioxide having controlled density.

7 Claims, No Drawings METHOD OF MAKING URANIUM DIOXIDE BODIESCONTRACTUAL ORIGIN OF THE INVENTION The invention described herein wasmade in the course of, or under, a contract with the UNITED STATESATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to a method formaking uranium dioxide bodies. More specifically, this invention relatesto a method for controlling the sintered density of uranium dioxidebodies.

One of the major problems of the atomic energy industry is thedevelopment of a fuel which can withstand the high temperaturesprojected for the reactors of the future. It is generally not possibleto get the desired performance from known fuels due to the dimensionalinstability of the fuels. Evidence is readily available which shows thatat a relatively low temperature, i.e., 500 to 1,100 C., uranium'dioxidefuels grow at a rate of about 0.6 to 0.8 volume percent per fissions percm. This growth or swelling is attributed to the formation of twofission products in the lattice of the solid fuel for each fissionevent. Many of these fission prod ucts are formed as gases whichaccumulate as tiny bubbles in the fuel. These bubbles are initiallyformed at high pressures and the rate at which they swell is a functionof the temperature and strength of the fuel. At still highertemperatures, gas mobility increases sufficiently that, after a largeamount of swelling has occurred, gas generation is balanced by gasrelease and swelling slows or ceases. One means for eliminating at leasta part of the swelling problem is to provide porosity by decreasing thedensity of the fuel so that the fission gases can readily escape intobuilt-in void spaces in the fuel element before they can damage the fuelelement by causing it to swell.

One method currently employed commercially to control the density so asto thus provide internal porosity within the uranium dioxide fuel is tomix uranium dioxide powder having high ceramic activity with uraniumdioxide powder of low ceramic activity before compacting and sinteringthe powders. The term ceramic activity" refers to a uranium dioxidepreparation which sinters readily to high density. Such material is thencalled an active" oxide. By this method fuel which is less dense than isnormally attainable with the use of the active" form of uranium dioxidealone can be produced. However, by this method it is difficult to obtainreproducible results and difficult to control the degree and uniformityof the porosity due to differing ceramic activities" of the forms ofuranium dioxide making up the mixture. In addition, the active" formwhich is usually very finely divided hyperstoichiometric uranium dioxideis highly pyrophoric and must be handled very carefully. This active"form can be made somewhat passive and still retain its sinterability.However, variables in the preparation, in the storing and in the passivetreatment of the active" oxide often result in variableoxygen-to-uranium ratios. Since the sinterability of the oxide dependson the degree of fineness of the oxide and on this oxygen-to-uraniumratio in the green compacted oxide, undesirable variations in sintereddensities of the final oxide can result.

Shapes of this material are also useful as crucibles certain chemicalreactions are important.

SUMMARY OF THE INVENTION We have developed a method for making sintereduranium dioxide bodies and'shapes, which can also be used in fuelelements, in which the final sintered density of the uranium dioxide iscontrolled by mixing U 0 with a predetermined amount of carbon to form acharge, the amount of carbon being determined by the final densitydesired, pressing the charge into a green body and heating the greenbody in an inert or slightly oxidizing atmosphere whereby the U 0 isreduced to hyperstoichiometric uranium dioxide and sintered to thedesired density. This produces a body useful wherever stoichiometry ofthe uranium dioxide is not important.

If the sintered body is to be used as a nuclear reactor fuel, the bodymay be heated in a hydrogen atmosphere to reduce the hyperstoichiometricuranium dioxide to stoichiometric uranium dioxide.

This method is also useful in preparing mixed oxide fuel of uraniumdioxide and plutonium dioxide which is presently the projected fuel forthe first generation liquid metal fast breeder reactor.

It is one object of this invention to produce dense uranium dioxidebodies.

It is another object of this invention to provide a method for producinguranium dioxide bodies having controlled density.

It is still a further object of this invention to provide a method forcontrolling the density of uranium dioxide bodies.

It is still a further object of this invention to provide a method forproducing sintered uranium dioxide bodies having controlled density inthe range from about 77 to 94 percent of the theoretical density.

It is another object of this invention to provide a method of producingsintered uranium dioxide bodies for use as nuclear reactor fuel.

It is still another object of this invention to provide a method forcontrolling the density of sintered uranium dioxide bodies for use innuclear reactor fuel elements without requiring a ceramically activeform of uranium dioxide as a starting material.

Finally, it is the object of this invention to provide a method forcontrolling the density of uranium dioxide plutonium dioxide bodies foruse in nuclear reactor fuel elements.

DESCRIPTION OF THE PREFERRED EMBODIMENT These and other objects of theinvention may be met by mixing carbon with U 0 in a mole ratio of from0.1 to 1.0 mole of carbon to l mole of U 0 to form a charge, the amountof carbon depending upon the final density of uranium dioxide which isdesired in the fuel or compact, compressing the charge into a green bodyat a pressure of at least about 10,000 lbs/in and slowly heating thegreen body in an inert or slightly oxidizing atmosphere to a temperatureabove about l,200 C. and maintaining this temperature for a period oftime sufficient to sinter the body, thereby first reducing the U 0 tohyperstoichiometric uranium dioxide, and sintering the body to thedesired density. if the sintered body is to be used as a nuclear reactorfuel, it may be subsequently heated in a hydrogen atmosphere to reducethe hyperstoichiometric uranium dioxide to stoichiometric uraniumdioxide.

As used herein, uranium dioxide will refer to hyperstoichiometricuranium dioxide unless it is stated that it has been reduced, in whichcase it will be stoichiometric or hypostoichiometric.

The hyperstoichiometric uranium dioxide as described herein has theformula U where x about 0.05 to 0.6. The amount ofx depends upon theamount of carbon which is mixed with the U 0 before the mixture isreduced. Thus, where no carbon is present, the U 0 will reduce to aboutUO and when the mixture contains about 1 mole of carbon per mole of U 0the U 0 will reduce to about UO Uranium dioxide shapes having thegreatest density are achieved with a mole ratio of about 0.7 mole carbonper mole of U 0 which reduces to the uranium dioxide having the greatestceramic activity.

The charge is prepared by thoroughly mixing U 0 and carbon. It isimportant that the U 0 used in this method be finely divided so that itwill be reduced to a uranium dioxide which is active or has a highceramic activity if uranium dioxide bodies having a high sintereddensity are desired. When lower density sintered uranium dioxide bodiesare desired, a less finely divided U 0 may be used with a proper moleratio of carbon to yield the uranium dioxide shape.

Finely divided U 0 can be prepared by several methods using differentstarting materials. For example, production-grade U0 can be used as astarting material by mixing it with water and grinding to dissolve anywatersoluble uranium compounds contained therein and to form a slurry.Excess ammonium hydroxide is added to precipitate any soluble uranium asammonium diuranate (ADU), the water is removed and the U0 and ADU areheated at a low temperature to decompose the mixture to a U 0 which isfinely divided. Finely divided U 0 can also be prepared by the thermaldecomposition of ADU alone in air or by comminution of a coarse U 0 Itis preferred that the carbon to be mixed with the U 0 is in the form ofpowdered graphite which acts as a binder to hold the charge in thedesired shape and also acts as a lubricant for the die. The mole ratioof carbon to U 0 depends upon density requirements. In order to obtainthe maximum density of uranium dioxide by this method, the mole ratio ofcarbon to U 0 may range from 0.68 to 0.72 with a ratio of 0.70preferred. Mixtures of U 0 and carbon containing over about 1.0 mole ofcarbon will result inthe presence of small amounts of carbon in thefinal compact by the heating procedure outlined here and will lower thefinal density. ln Table I below are given a number of densities whichwere obtained by varying the mole ratio of carbon to U 0 The sintereddensities were obtained from charges containing U 0 (obtained by thethermal decomposition of ADU in air) and graphite and were pressed at31,000 lbs/in and sintered in carbon dioxide for 2 hours at 1,400 C.

TA BLE l Sintered Density of Charge Composition Theoretical DensityAlthough pressures of 31,000 lbs/in were used to The pressed compactsare sintered in an inert or slightly oxidizing atmosphere attemperatures above about l,200 C. for a period of time sufficient tosinter the body. Although the maximum sintering time is not critical,the time does vary with the temperature. For example, it was found thata period of about 2 hours at a temperature of 1,400 C. was sufficient toobtain essentially the greatest density of each composition andsintering times of up to about 5 hours did not increase the finalproduct density.

Although an inert atmosphere such as argon, helium or nitrogen willprovide satisfactory densities, the use of an atmosphere which isslightly oxidizing such as carbon dioxide is preferred because thestoichiometry of the uranium oxide is more nearly optimized andmaintained as temperature increases and the densities obtained therewithare slightly greater than are attainable in an inert atmosphere.

If the sintered body is to be used in a reactor fuel element, it will benecessary to completely reduce the uranium dioxide to U0 since theuranium oxide, after sintering, is generally slightlyhyperstoichiometric. This is readily accomplished by subsequentlyheating the body in a hydrogen atmosphere at a temperature above aboutl,200 C. a period of time sufficient to accomplish the reduction andcooling the body in the same atmosphere to prevent any pickup of oxygenfrom the air. It was found that about one-half hour at a temperature ofabout 1,400 C. was sufficient to complete the desired reduction.

EXAMPLE 1000 grams of production-grade uranium dioxide were dissolved inconcentrated nitric acid (HNO to yield a uranyl nitrate solution (UO (NO-6l-l- O) containing about grams of uranium per liter of solution. Thissolution was then heated to about 60 C. and ammonium hydroxide(Ni-1,011) added to precipitate the uranium as ammonium diuranate[(Nl'h) U 0 Enough ammonium hydroxide was added to completelyprecipitate all the uranium and increase the pH of the solution to about9.0. After the ammonium diuranate was filtered, washed and dried, it waspowderized and converted to U 0 by heating in air at 800 C. Portions ofthis U 0 were then used in subsequent sintering experiments.

Thirteen charges containing variable carbon to U 0 mole ratios werefirst prepared. This was accomplished by thoroughly mixing 50 grams of U0 and enough carbon (graphite powder) to yield the desired carbon to U 0mole ratio. Table 11 includes a summary of the weights of oxide andcarbon used in each charge and the resulting carbon toU;,O,.rnole ratioin each charge.

Next, green bodies were prepared by cold-pressing 6 to 7 grams from eachcharge at 31,000 lbs/in in a double-action steel die. These green bodieswere then slowly heated in a carbon dioxide atmosphere through thereaction range up to about 900 C. and further heated to l,400 C. andheld at this temperature for 2 hours. A body prepared from the same U 0with no carbon addition was pressed and sintered at the same TABLE ll 2.C/U,O Sintered Density ratio% of Theoretical 0.073.88 0.17700 0280.000383.20 0.48615 0.58937 0692.25 0793.80 0.893.25 0.992.50 L091 .801.190.70" 1.29030 1.39000" Sample Wt. of Wt. of

No. U 0 used carbon used Blank 50 grams *These compacts also containedresidual carbon.

The untreated U 0 yielded a final sintered density of about 74 percentof theoretical. Increasing the carbon to U 0 mole ratio through 0.7, in0.1 mole increments, produced subsequent increases in the final sintereddensities. Charges with carbon to U 0 mole ratios above 0.7, however,yielded successively lower sintered densities. Sintered compactsprepared from charges containing carbon to U 0 mole ratios of greaterthan 1.0 would appear to have no significant application since residualcarbon was present after sintering. However, higher sinteringtemperatures with vacuum after the l,400 C. treatments could yieldhypostoichiometric uranium dioxide with these compositions.

As can be seen, the process of this invention provides a method forproducing uranium dioxide bodies in which the final density of thebodies can be readily controlled from about 77 percent to about 94percent of theoretical density. Greater porosities can also be achievedby substituting a coarser form of U 0 for some of the lU O, used in theabove example.

The method of this invention is also readily adaptable to preparingmixed oxide compacts of plutonium dioxide and uranium dioxide byaddition of the desired amount of plutonium dioxide to the carbon U 0while maintaining the same carbon to U 0 ratio.

It will be understood that the invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

We claim:

ll. A method of making a uranium dioxide body having controlled finaldensity of from about 77 to about 94 percent of the theoretical densitycomprising: mixing finely divided U 0 with a predetermined amount ofcarbon to prepare a charge, the amount of carbon varying from a ratio of0.1 to 10 moles of carbon to 1 mole of U 0 and being determined by thefinal density desired; pressing the charge into a green body; andheating the green body in an inert or slightly oxidizing atmosphere to atemperature of from about 1,200 to about l,500 C. until the U 0 isreduced to hyperstoichiometric uranium dioxide and sintered, therebyforming a uranium dioxide body having controlled den- Sity.

2. The method of claim It wherein the carbon is graphite and the U 0 andgraphite charge is pressed at a pressure of at least 10,000 lbs/in toform a green body.

3. The method of claim 2 wherein the atmosphere is selected from thegroup consisting of argon, helium, nitrogen and carbon dioxide.

4!. The method of claim ll comprising the additional step of heating theuranium dioxide body in a hydrogen atmosphere, thereby completelyreducing the body to stoichiometric uranium dioxide.

5. The method of claim d wherein the body is heated to a temperatureabove about l,000 C. for a period of time sufficient to reduce the bodyto stoichiometric uranium dioxide.

6. The method of claim 5 wherein the body also contains plutoniumdioxide.

7. A method of making a uranium dioxide body for use in a fuel elementwherein the body has a controlled final density of from about 77 toabout 94 percent of the theoretical density comprising: mixing U 0 withgraphite in a ratio of 0.1 to 0.7 mole graphite to 1 mole of U 0 to forma charge; the amount of graphite in the charge being determined by thedesired final density; pressing the charge into a green body; heatingthe green body in a carbon dioxide atmosphere up to a temperature ofl,350 to 1,500 C. for at least 2 hours whereby the U 0 is reduced tohyperstoichiometric uranium dioxide and sintered; and heating thesintered body under a hydrogen atmosphere at a temperature of l,3S0 tol,500 C. for at least onehalf hour, whereby said hyperstoichiometricuranium dioxide is reduced to stoichiometric uranium dioxide, therebyforming a uranium dioxide fuel element having controlled density.

i W *3 m =3

2. The method of claim 1 wherein the carbon is graphite and the U3O8 andgraphite charge is pressed at a pressure of at least 10, 000 lbs/in2 toform a green body.
 3. The method of claim 2 wherein the atmosphere isselected from the group consisting of argon, helium, nitrogen and carbondioxide.
 4. The method of claim 1 comprising the additional step ofheating the uranium dioxide body in a hydRogen atmosphere, therebycompletely reducing the body to stoichiometric uranium dioxide.
 5. Themethod of claim 4 wherein the body is heated to a temperature aboveabout 1,000* C. for a period of time sufficient to reduce the body tostoichiometric uranium dioxide.
 6. The method of claim 5 wherein thebody also contains plutonium dioxide.
 7. A method of making a uraniumdioxide body for use in a fuel element wherein the body has a controlledfinal density of from about 77 to about 94 percent of the theoreticaldensity comprising: mixing U3O8 with graphite in a ratio of 0.1 to 0.7mole graphite to 1 mole of U3O8 to form a charge; the amount of graphitein the charge being determined by the desired final density; pressingthe charge into a green body; heating the green body in a carbon dioxideatmosphere up to a temperature of 1, 350* to 1,500* C. for at least 2hours whereby the U3O8 is reduced to hyperstoichiometric uranium dioxideand sintered; and heating the sintered body under a hydrogen atmosphereat a temperature of 1,350* to 1,500* C. for at least one-half hour,whereby said hyperstoichiometric uranium dioxide is reduced tostoichiometric uranium dioxide, thereby forming a uranium dioxide fuelelement having controlled density.